Abstract
In recent years, the prevalence of obesity has significantly increased in developed countries, a trend that has been just as apparent in France as elsewhere. Obesity may predispose to osteoarthritis, which may subject a high percentage of patients to shoulder surgery. Interscalene block remains one of the most efficient technique for postoperative analgesia after shoulder surgery. However, there are no specific guidelines or discussions in the literature about the use and management of interscalene block for obese patients. No study has yet specifically assessed obesity relative to interscalene block. Regional anesthesia offers certain advantages in obese patients: minimal airway intervention, improved postoperative analgesia, and decreased opioid consumption. In obese patients undergoing shoulder surgery, the goal of postoperative pain management is to provide comfort, early mobilization, and improved respiratory function without the worry of inadequate sedation and respiratory compromise. The ongoing debate on the relevance of obesity, for example in terms of obstructive sleep apnea syndrome, obesity hypoventilation syndrome, and other related respiratory disturbances, in relation to the choice of analgesic techniques, especially interscalene block, patient-controlled intravenous analgesia and patient monitoring, demands large-scale, well-designed studies to resolve it. Nevertheless, obesity per se should not dissuade patients from undergoing shoulder surgery under interscalene block.
Keywords
interscalene block; obesity; pain, postoperative; regional anesthesia; respiratory dysfunction; ultrasound guidance;
1. Introduction
Severe postoperative pain is frequently observed after major shoulder surgery, particularly within the first 48 hours.1 Adequate pain management in this clinical setting is important not only to improve the patient's well-being and rehabilitation, but also to reduce hospital stay and facilitate a return to normal life.1
Over the past few decades, obesity has become one of the most important public health problems globally. Its prevalence has significantly increased in recent years, and this trend has been just as apparent in France as elsewhere. Recently, the French Institute of Statistics and Economical Studies reported a rise in its prevalence among adults in France from 5.3% to 9.8% for men and 5.3% to 10.2% for women between 1981 and 2003.2 This epidemic situation may have important implications for anesthesiologists3 as obese and morbidly obese patients have a reduced functional residual capacity.4 They are at high risk for difficult airway management, cardiopulmonary dysfunction, acid aspiration, and markedly increased perioperative morbidity and mortality.3
The use of interscalene block (ISB) for shoulder surgery in obese patients as a substitute for general anesthesia is a tempting prospect. It offers several advantages, such as decreased postoperative nausea and vomiting and reduced postoperative pain by continual blockade with long-acting local anesthetics. However, attempts to successfully achieve this block may be doomed to failure because of the patient's size.4 Nevertheless, ISB remains one of the most efficient techniques for postoperative analgesia after shoulder surgery.5
Phrenic nerve involvement may restrict the use of ISB in patients with limited respiratory function,6 especially mildly and morbidly obese patients.7, 8, 9 To date, no study has evaluated the role of ISB in the obese population. This article is a review of the current literature.
2. General considerations related to obesity
2.1. Definition of obesity
Obesity is a condition of excessive body fat. The name is derived from the Latin word obesus, which means “fattened by eating.” The difference between normality and obesity is arbitrary, but an individual must be considered obese when the amount of fat is increased to such an extent that physical and mental health is affected and life expectancy is reduced.8, 10
In clinical practice, several criteria have been proposed to exactly define obesity, namely height and weight indexes, a calculation ratio relating the actual and ideal weight of the patient, and the body mass index (BMI).11 However, BMI is used by almost all studies as a measure of overall obesity.9, 12, 13 BMI is divided in five categories4: normal weight, <25 kg/m2; overweight > 25–30 kg/m2; obesity > 30 kg/m2; morbid obesity > 35 kg/m2; and supermorbid obesity > 55 kg/m2. Although BMI is not an ideal measure of risk, it is the most useful of the currently available markers.4
2.2. Etiology
Obesity is a complex multifactorial disease, but, put simply, it occurs when net energy uptake exceeds net energy consumption over a long period of time. Other factors implicated in the etiology of obesity are genetics, ethnic influence (Afro-Americans and Hispanic Americans, for example, being more prone to the disorder than white Americans), socioeconomic factors, endocrine abnormalities, and intake of certain drugs such as corticosteroids.9, 10
Body fat distribution also plays an important role. In males fat distribution is central, whereas in females it is more frequently peripheral.9 However, a central distribution of adipose tissue is associated with a higher incidence of co-morbidities, especially respiratory, metabolic, and cardiovascular.9, 10
2.3. Respiratory function
Simple obesity uncomplicated by upper or lower airway obstruction [obstructive sleep apnea syndrome (OSAS) or obesity hypoventilation syndrome (OHS)] generally produces mild effects on respiratory function.11
Increased BMI is not a predictor of difficult laryngoscopy: it has been reported as impossible to view the vocal cords in 1% of such cases, and failed intubation has been reported in about 0.35% of subjects.11 Approximately 5% of morbidly obese patients will have OSAS or OHS.10 These co-morbidities are associated with a higher risk for difficult intubation.11 Mass loading of the thoracic and abdominal components on the chest wall in awake obese patients in the supine position causes abnormalities of lung volume and gas exchange.10
When obese patients are free of chronic pulmonary disease, only a major degree of increase in body weight will significantly affect pulmonary function.13, 14 Obesity induces a reduction in thoracic compliance and is associated with reduced functional capacity when the person is seated, particularly with a reduced expiratory reserve volume. These parameters are further reduced in the supine position, mainly due to the gravitational effect of the large abdomen, with further displacement of the diaphragm into the thoracic cavity.12 This condition and other risk factors contribute to acute postoperative respiratory dysfunction in obese patients. Furthermore, obese patients have a modest defect in gas exchange preoperatively, with a reduction in Pao2 and an increase in alveolar to arterial oxygen difference and shunt function.10 Accordingly, phrenic nerve involvement and hemidiaphragmatic paralysis may contribute to a further deterioration in gas exchange. In this case, high inspired fractions of oxygen are required to maintain adequate arterial oxygen tensions.
2.4. Cardiovascular function
Cardiovascular disease, expressed as ischemic heart disease, hypertension, and cardiac failure, dominates morbidity and mortality in the obese population. A Scottish survey10 found that the prevalence of any cardiovascular disease in the obese population was approximately 37% in adults with a BMI > 30 kg/m2, 21% with a BMI of 25–30 kg/m2, and only 10% with a BMI < 25 kg/m2. All morbidly obese patients presenting for any type of anesthesia should be extensively investigated for cardiovascular complications.9, 10 Furthermore, the all-cause mortality has shown a J-shaped relationship between BMI and the overall mortality.9 This indicates that all-cause mortality is increased with higher body weight.9
2.5. Obesity and ultrasound guidance
The anthropometric changes associated with obesity can make it technically difficult to perform peripheral nerve blockade.8, 15 In a prospective study, Cotter et al reported that a BMI > 25 kg/m2 was an independent risk factor for failure of the technique.15
Moreover, ultrasound is the imaging modality that could be mostly affected by obesity.16 Excess of subcutaneous fat typically impairs image quality by attenuating the ultrasound signal. There is a relationship between fat thickness and the amount of degradation of the ultrasound beam.16 Ultrasound experts have recommend that a probe of the lowest possible frequency must be used because ultrasound energy is most attenuated at high frequencies.16, 17 Thus, alteration of image quality may contribute to malpositioning of a needle or catheter, accounting for the high failure rate in terms of, for example, uncontrolled local anesthetic diffusion, intravascular injection, and local anesthetic toxicity.4, 8, 17 Although Cotter et al reported that obesity was associated with a statistically significant higher rate of failure and complications, related to local anesthetic dose or needle placement, the overall complication rate remained low.15 This suggests that obesity per se should not prevent patients from undergoing ISB.
2.6. Obesity and diabetes
The relative risk of developing type 2 diabetes increases steeply with increasing BMI, and patients with a BMI > 35 kg/m2 have a 93 times higher risk for developing diabetes.9 Although type 2 diabetes may cause peripheral neuropathy, obese patients may present subclinical peripheral nerve impairment associated with impaired glucose tolerance and insulin resistance.18 Some authors have advanced the double-crush hypothesis, theorizing that nerves are compromised at one site but may become susceptible to injury at another.19, 20 This hypothesis cannot be applied to a generalized neuropathy, and recent studies seem to dismiss this theory.21 Accordingly, obese patients are not at higher risk for neuropathy after peripheral nerve block than the normal-weight population.
3. Specific considerations related to obesity and ISB
The literature reports very few studies specifically assessing obesity and ISB. Few studies report high failure rate after regional anesthesia in obese patients.3, 8 Table 1 illustrates dyspnea, hoarseness and Horner syndrome reported after ISB or supraclavicular block in nonobese patients.22, 23 Table 2 illustrates pulmonary function in nonobese patients after ISB.6, 24, 25, 26, 27
Obese patients are predisposed to osteoarthritis,6 which may explain why a high percentage of patients are subjected to shoulder surgery. Pain management after shoulder surgery in obese patients is of great importance, not only for the patient's comfort, but also for the outcome.28 The pathophysiology of obesity, typical co-morbidities, and the high prevalence of OSAS or OHS among obese patients make safe analgesic management difficult.29
Furthermore, pain, postoperative administration of opioids, and ISB in the obese population may increase the incidence of postoperative respiratory complications. Therefore, acute pain management after shoulder surgery raises several problems: the choice of the safest and most suitable analgesic technique, appropriate postoperative monitoring, and the prevention of respiratory complications.29
Performing ISB on obese patients may offer many important advantages compared with general anesthesia, including the possibility of avoiding airway manipulation, the lack of a cardiodepressant action of inhaled anesthetics, and avoidance of the respiratory-depressant action of opioids.17 Nevertheless, phrenic nerve involvement associated with reduced thoracic compliance after ISB may cause respiratory dysfunction and hypoxia. In addition, these patients are at high risk for ISB failure, and a failed or incomplete ISB may require conversion to general anesthesia with endotracheal intubation. However, postoperative patient-controlled intravenous analgesia with opioids may also involve poor respiratory function leading to hypoxia. There is paucity of evidence-based recommendations for postoperative analgesia in the literature. The majority of published papers dealing with anesthesia and analgesia for obese patients are mainly expert opinion.29
Patients with a BMI > 30 kg/m2 who present one of the following risk factors must therefore go through an exploration of their respiratory function and polysomnography before defining the postoperative analgesic strategy:
•American Society of Anesthesiologists physical status > 3
•Collar size > 6.5 inches (42 cm)
•Known history of OSAS or OHS
•Dyspnea on moderate effort
•Diurnal somnolence despite adequate sleep
•Smoking with chronic bronchitis
•Tonsils touching each other
•Loud snoring during sleep.
3.1. Postoperative analgesic strategy for shoulder surgery
In obese patients undergoing shoulder surgery, the goal of postoperative pain management is to provide comfort, early mobilization, and improved respiratory function without causing inadequate sedation and respiratory compromise. Nevertheless, advice on general management includes multimodal analgesic therapy and some other supporting techniques.
Despite the risk of phrenic nerve involvement, ISB remains highly efficient in reducing opioid-related complications.29 Borgeat et al demonstrated no difference with regard to respiratory function and hemidiaphragmatic excursion between patients receiving intravenous morphine analgesia or continuous ISB.1
Therefore, in order to reduce phrenic nerve involvement after ISB, the following essential points are suggested: (1) ultrasound guidance; (2) a low volume of local anesthetic on initial injection and low-volume continuous perfusion; (3) injection through the catheter; and (4) needle and catheter placement at the C7 root.
These factors may ensure correct catheter insertion and reduce local anesthetic diffusion to the phrenic nerve through the anterior scalene muscle and its fasciae, or directly to the paravertebral nerve root from which the phrenic nerve is derived.6, 24, 30, 31 Most importantly, it should be noted that low-volume injection of local anesthetic for ISB results in no difference in postoperative analgesia compared with high-volume injection.6, 24 Multimodal supplementation of intravenous analgesics [acetaminophen, non-steroidal anti-inflammatory drugs (NSAIDs), and nefopam] should be used systemically when acceptable.19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32
3.2. Adjunct drugs and obesity
The use of adjunct drugs such as ketamine, pregabalin, and alpha-2-adrenergic agonists should theoretically reduce opioid consumption. However, there are a limited number of clinical studies with these agents in obese patients. Despite this, an appropriate systemic analgesic regimen for postoperative pain control may be best to include pregabalin premedication, paracetamol, and an NSAID when ISB is used in association with general anesthesia for shoulder surgery.29
3.3. Oxygenation
Although studies in the literature are insufficient to evaluate the effects of postoperative supplemental oxygen administration to these patients, it is rational to support the use of supplemental oxygen in the postextubation period to improve patients' oxygen saturation. There is also insufficient literature to evaluate the effect of continuous positive airway pressure (CPAP) or noninvasive positive-pressure ventilation (NIPPV) on the postoperative respiratory status of obese patients. However, the literature supports the efficacy of CPAP in nonperioperative settings.30 Accordingly, the use of CPAP or NIPPV may improve the patient's respiratory status postoperatively.
3.4. Patient positioning
There is some evidence in the literature that the lateral, prone, and sitting positions improve Apnea–Hypopnea Index scores in obese patients in nonperioperative settings, but there is insufficient information in the literature to provide guidance for the postoperative period. In postoperative settings, the supine position should thus be avoided.32 In our experience, obese patients best tolerate the sitting position after shoulder surgery.29
3.5. Monitoring
There are no evidence-based data on postoperative monitoring in obese patients. However, because of the risk of respiratory failure, pulse oximetry, one of the most important techniques, and end-tidal carbon dioxide measurement can provide important information.29, 30, 31, 32
Another known complication in obese patients is hypertension, but noninvasive blood pressure monitoring can be difficult, and the results may not be accurate. Intra-arterial blood pressure measurement must therefore be discussed on individual basis.29
3.6. Mobilization
Early postoperative mobilization is very important in obese patients in order to avoid pressure ulceration, pulmonary embolism, and deep vein thrombosis. Furthermore, the beach chair position and aggressive physiotherapy are widely recommended.29, 30, 31
4. Conclusion
Obese patients scheduled for shoulder surgery may be at high risk for postoperative complications. To date, there are neither evidence-based data nor studies on the use of ISB for postoperative analgesia after shoulder surgery in the obese population. However, the overall consensus supports the concept of multimodal analgesia and the preferential use of regional anesthetic techniques. The ongoing debate on the relevance of obesity, OSAS, OHS, and other related respiratory disturbances to the choice of analgesic techniques, especially ISB, and monitoring needs to be resolved by appropriate large, well-designed studies. Nevertheless, obesity per se should not prevent patients from having ISB for shoulder surgery.